Non-Invasive Compressibility and In Situ Density Testing of a Fluid Sample in a Sealed Chamber

ABSTRACT

In situ density and compressibility of a fluid sample are determined for a fluid sample collected downhole. The density and compressibility of the fluid sample is determined by measuring a distance to a piston contained within the sample chamber using an external magnetic field sensor that senses a magnetic field emanating from a magnetic provided on the piston internal to the sample chamber. The testing is performed quickly and at the surface in a noninvasive fashion (i.e., without opening the sample chamber).

BACKGROUND

An application of formation fluid testing is to confirm the mobile fluidphase in the reservoir. This determination can be important inreservoirs in which there is significant uncertainty about the formationwater salinity. This situation is further complicated in poorpermeability reservoirs where there can be a long oil-water transitionzone. Defining the mobile fluid phase down the transition zone can beachieved by sampling with, for example, a pump-out wireline formationtester (PWFT). This tool incorporates downhole sensors to analyze thefluid while pumping, the results of which are used to determine when andhow to sample the formation fluid. The fluid samples are received intosample chambers.

After the sample chambers are retrieved to the surface, the chamberstypically are sent to a lab for transfer of the sampled fluid anddetailed analysis. Often, there is a long delay between retrieving thesample chambers and obtaining the analysis results; at times the delaycan be on the order of weeks. Such delays are undesirable given the highcosts associated with drilling operations.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of exemplary embodiments of the invention,reference will now be made to the accompanying drawings in which:

FIG. 1 illustrates an embodiment of sample chamber;

FIG. 2 illustrates another embodiment of a sample chamber;

FIG. 3 illustrates yet another embodiment of a sample chamber;

FIG. 4 illustrates a further embodiment of a sample chamber;

FIG. 5 provides a block diagram of test system in accordance with thepreferred embodiments;

FIGS. 6 a and 6 b depict magnetic fields associated with a magnetprovided on a piston internal to the sample chamber;

FIG. 7 shows a method embodiment in which a fluid sample is collectedand tested at the surface for in situ density and compressibility;

FIG. 8 shows an illustrative method embodiment for determining in situdensity of a fluid sample in a sealed chamber;

FIG. 9 shows another method embodiment of the determination of in situdensity of the fluid sample; and

FIG. 10 shows a method embodiment for determining fluid samplecompressibility.

DETAILED DESCRIPTION

The following discussion is directed to various embodiments of theinvention. Although one or more of these embodiments may be preferred,the embodiments disclosed should not be interpreted, or otherwise used,as limiting the scope of the disclosure, including the claims. Inaddition, one skilled in the art will understand that the followingdescription has broad application, and the discussion of any embodimentis meant only to be exemplary of that embodiment, and not intended tointimate that the scope of the disclosure, including the claims, islimited to that embodiment.

The embodiments disclosed herein are directed to surface testing of asealed sample chamber containing a fluid sample obtained downhole fromthe formation. The fluid sample is received into the sample chamber andheld at in situ pressure inside the sample chamber (i.e., pressure ofthe fluid while in the formation). The surface testing is relativelyquick, noninvasive (i.e., testing is performed without opening thesample chamber) and includes a determination of either or both of thefluid sample's in situ density and compressibility. The testing isperformed in an automated fashion (i.e., with little or no humaninvolvement) by a computer-operated testing system. The testing isperformed without opening the sample chambers. Once the testing iscomplete at, for example, the rig site, the sample chambers can be sentto a lab for further testing.

The sample chambers for which the surface testing is performed generallycomprise a cylindrical container containing one or more pistons thatseal against the inner wall of the container and can be moved from oneend of the container to the other. Some sample chambers have only asingle piston while other sample chambers have two pistons. Some samplechambers include a buffer fluid (air, water, nitrogen, etc.). Theoperation of the sample chambers varies with the various types ofchambers and the embodiments disclosed herein for determining densityand compressibility are effected by the various chamber designs.Accordingly, the following discussion includes an overview of varioussample chamber designs, followed by an explanation of the preferredembodiments of a testing system.

Four illustrative sample chambers are shown and discussed below withregard to FIGS. 1-4. FIG. 1 illustrates a downhole fluid sample chamber10 including a sleeve-shaped cylinder 12 which forms an interior fluidcompartment 18 therein. Cylinder 12 is substantially closed by an upperend cap 14 and lower end cap 16, with compartment 18 between the endcaps and initially containing low pressure air that fills compartment 18through valve 34 which is then closed to trap the air in the compartment18. A separator piston 20 (also referred to herein as a “fillingpiston”) movably sealed to the cylinder 12 initially is positioned inthe upper end of the cylinder adjacent the end cap 14. When the samplechamber 10 is used in a well to collect a sample, and in response to thefluid sample pressure being greater than the air pressure in chamber 18,the separator piston 20 moves downward, thereby compressing the airwhich causes the pressure of the air to increase. The separator piston20 movies downward until the air pressure on the lower side of thepiston in compartment 18 is substantially equal to the pressure of thefluid sample on the upper side of the piston (between the piston andupper end cap 14). With the pressures substantially equal, the piston 20will reside between the upper end cap 14 and lower end cap 16, andgenerally slightly above the lower end cap 16.

The upper end cap 14 includes a fluid passageway 22 therein fortransmitting formation fluid into the cylinder 12 and to the top side ofthe separator piston 20. An isolation valve 36 is located along the flowpath 22 in the upper end cap 14. Valve 36 is closed once the fluidsample is obtained. A fluid line 24 extends from the upper end cap 14 tothe formation of interest 30, and an electronic flow line control valve26 is positioned along the flow line 24 for controlling the fluid flowfrom the formation to the sampling cylinder. FIG. 1 further illustratesan annular packer element 28 for sealing engagement with the face offormation 30, so that formation fluid passes through the center ofpacker element 28 and to the flow line 24, and then to the cylinder 12.The lower end cap 16 also has a flow line 32 therein which communicatesbetween the compartment 18 and exterior of the cylinder 12, with anormally closed valve 34 controlling the release of fluid along the flowline 32.

A valve 25 extends from line 24. The valve 25 remains closed when thetool is downhole. A pressure gauge (not shown) may be fluidly connectedto the outlet of the valve 25 at the surface, and the valve 25 brieflyopened to determine the pressure of the test fluid in the cylinder 12.

The fluid compartment 18 within the cylinder 12 thus initially serves asan air chamber for atmospheric air. To collect a formation fluid sample,the flow line control valve 26 is open to introduce formation fluid intothe interior of the cylinder 12, thereby forcing the piston 20 downward.As the piston 20 moves downward toward the lower end cap 16, the airbetween piston 20 and the lower end cap 16 becomes increasinglycompressed. Formation fluid at in situ (formation) pressure, fills thecompartment 18 between piston 18 and upper end cap 14. Once the pressureof the compressed air below the piston 20 and the fluid sample above thepiston 20 are at substantially the same pressure, the piston 10 stopsmoving and the flow line control valve 26 may be closed, therebytrapping the collected fluid sample within the cylinder 12.

FIG. 2 illustrates an alternate sample chamber 10 b wherein water,ethylene glycol, oil, or another selected incompressible fluid may beused as a buffer fluid. The flow line 24 may be as discussed above inFIG. 1 to provide fluid communication with the interior of the cylinder12. In the embodiment of FIG. 2, an upper packer 27 and a lower packer29 are used to isolate fluid within formation 30 from the remainder ofthe wellbore. Any of the embodiments shown in FIGS. 1-4 may be used withthe packer element 28 as shown in FIG. 1 or the straddle packers 27 and29 shown in FIG. 2. Central member or choke sub 15 is fixed within thecylinder 12. The separator piston 20 initially may be positionedsubstantially adjacent the upper end cap 14, with the space between thepiston 20 and the central member 15 being filled with water. The spacebelow the member 15 and the lower end cap 16 may initially be filledwith air at atmospheric pressure. A vent tube 46 may pass water fromabove the central member 15 to an area below the central member 15 andthrough a choke 44 positioned along the flow path 45 within the member15. A bypass valve 48 may be used at the surface to recycle the chamberand for piston management. Flow line 50 fluidly connects the chamberbelow the central member 15 to the exterior of cylinder 12. Both theflow line 32 in lower end cap 16 and the outlet from flow line 50 in thecentral member 15 may each be closed by a plug 33.

When the flow line control valve 26 is open, formation pressure acts onthe separator piston 20 and forces the buffer fluid, which, as notedabove, may be water, ethylene glycol, oil or another selectedincompressible liquid, through the restriction or choke 44, therebyestablishing a threshold flowing pressure at which the formation fluidenters the chamber. The chamber 56 below the central member 15 and abovethe lower end cap 16 may be referred to as a choke chamber. Formationfluid forces the water through the choke and into the air filled (or gasfilled) choke chamber 56, thereby compressing the air. Space 52 belowcentral member 15 may thus be compressed air, with the interface 54shown between the compressed air and the liquid. The separator piston 20continues to move downward until the pressure of the compressed air isapproximately equal to the pressure of the sample fluid from theformation. The flow line control valve 26 then may be closed to trap thecollected fluid sample within the cylinder 12.

Referring now to FIG. 3, an embodiment of yet another sample chamber 10c is shown. In this embodiment, a downhole pump 60 is included, with theinlet of the pump 60 connected to the downhole formation of interest 30and the outlet of the pump 60 connected to the cylinder 12. In suchembodiments, a tool string includes the pump 60 and multiple (e.g., 15)sample chambers 10 c. Each sample chamber 10 c has an associatedelectronically controlled valve 26 that, when opened, connects the pump60 to that particular sample chamber. Initially, the piston 20 may beprovided in the upper portion of compartment 18 adjacent the upper endcap 14. Valve 34 within the lower end cap 16 is open. Accordingly, thelower side of the piston 20 is exposed through open valve 34 to wellborefluid at hydrostatic pressure. Downward motion of the piston 20continues until piston 20 reaches its full extent of travel and restsagainst the upper surface 17 of the lower end cap 16. At that point,valves 34 and 36 are closed.

FIG. 4 illustrates yet another configuration for a sample chamber 10 dwhich utilizes a compressible gas (e.g., nitrogen) that is pressurizedto downhole conditions to compensate for sample contraction uponcooling. A pump 60 and valves 26 and 25 are provided as with the priorembodiment. The separator piston 20 thus is initially positionedadjacent the upper end cap 14, and nitrogen gas is contained in thespace 66 between the separator piston 20 and a separate charging piston64, which is sealed to cylinder 12 and does not pass fluid throughpiston 64. Wellbore fluid at hydrostatic pressure is exposed to thelower side of the charging piston 64 as valve 34 is open to providefluid flow along the flow line 32 in the lower end cap 16. The outletfrom the downhole pump 60 is directed to the cylinder 12, and as thecompartment 62 (between the separator piston 20 and upper end cap 14) isfilled with the fluid sample from the formation, the separator piston20, the nitrogen gas and the charging piston 64 are pushed downwarduntil the charging piston 64 reaches its full extent of travel, as shownin FIG. 4. Additional pumping moves the separator piston 20 furtherdownward, compressing the nitrogen charge. Once overpressurized to thedesired level, the flow line control valve 26 may be closed therebytrapping the collected sample within the sample chamber.

In each of the embodiments of FIGS. 1-4, the separator (filling) pistonincludes a magnet 76. The magnet 76 is supported on the separator piston20, and preferably positioned within the separator piston. Due in partto the corrosive nature of the fluids contained within the cylinder 12,the cylinder 12 and the piston 20 preferably are fabricated from a highnickel alloy, such as inconel 718 or a titanium alloy. Besides beingcorrosion resistant, these materials are relatively non-magnetic. Themagnet 76 within the piston 20 may be disk shaped, and typically may bean Al—Ni—Co or Sm—Co material. The magnet 76 is fixed coaxially withinthe separator piston 20 and is magnetized along its axis, which issubstantially coaxial with the axis of the cylinder 12.

The embodiments of FIGS. 3 and 4 include the use of downhole pump 60with the sample chambers. The downhole pump 60 preferably includes afilling sensor 61 such as a potentiometer that produces a signalindicative of the volume of fluid pumped by the pump 60 into the samplechamber. Once pumping ceases, the filling sensor's value is read andtransmitted to the surface via, for example, wireline or mudpulsetechniques for recording by a test system (discussed below). Theembodiments of FIGS. 1 and 2 do not use a pump and a different technique(discussed below) is used to determine the volume of the captured fluidsample. This latter technique can be used as well even in theembodiments in which a pump 60, and associated filling sensor, isavailable.

FIG. 5 shows a preferred embodiment of a test system 100. FIG. 5 alsoshows a sample chamber 10 (which can be any suitable sample chamber suchas any of the chambers 10 a-10 d of FIGS. 1-4) loaded into the testsystem 100. Test system 100 may be located, for example, at the surface.The sample chamber 10 may reside, for example, on a support structure.As shown, the test system 100 comprises a control unit 110 coupled to ahydraulic unit 120 and to a linear position device 130. The control unit110 may be, for example, a computer. The control unit 110 comprises aprocessor 112 coupled to a computer-readable storage medium 114 thatcontains executable software 118. The computer-readable storage medium114 may comprise volatile memory (e.g., random access memory) ornon-volatile storage (e.g., hard disk drive, flash storage, CD ROM,etc.). The software 114 is executed by processor 112 and, as such,causes the processor to perform, or at least initiate, some or all ofthe functionality described herein attributed to the test system 100and/or control unit 110. One or more input/output (I/O) devices 116 arealso included and coupled to the processor 112. Such I/O devices mayinclude, for example, a keyboard, a mouse, a touchpad, a display, etc.

The hydraulic unit 120 comprises a hydraulic pump that is connectable tothe sample chamber 10 via a hydraulic line 122. The hydraulic unit 120can vary the pressure inside the hydraulic line in accordance with asignal 119 from the control unit 110. The control unit 110 thus cancause the hydraulic unit 120 to increase or decrease the pressure in thehydraulic line. The content of the hydraulic line may be a gas such asnitrogen, but other suitable hydraulic gasses or fluids may be used aswell.

A pressure sensor 124 is provided on the hydraulic line 122. Thepressure sensor 124 produces an electrical signal 125 that isproportional to the pressure in the hydraulic line 122. Signal 125 isprovided to the control unit 110 which can monitor the pressure in thehydraulic line via the pressure sensor 124.

The linear position device 130 determines the location of the piston 20within the sealed sample chamber. The linear position device 130comprises a sensor locating device 131 and a magnetic field sensor 132which can move along or near the exterior surface of the sample chamber10 in the x-direction between one end 127 of the sample chamber and theother end 129. The magnetic field sensor 132 is sensitive to themagnetic field emanating from the piston's magnet 76. The magnetic fieldsensor 132 preferably comprises a Hall sensor, magnetoresistive sensor,fluxgate field sensor, induction coil sensor, induction coilgradiometer, or other suitable type of sensor. The sensor 132 may havesingle axis or multi-axis sensitivity. Further, the electrical signal123 from the magnetic field sensor 132 is provided to the control unit110.

The sensor locating device 131 is able to determine the position of themagnetic field sensor 132 and produce a signal 121 that encodes thesensor's position. The signal 121 is referred to as the position signal.The sensor locating device 131 determines the position of the sensor 132via any of a variety of techniques. For example, the sensor locatingdevice 131 may comprises a linear potentiometer, a laser distancesensor, an ultrasonic distance meter, a digital ruler, a draw wiresensor, etc.

In some embodiments, the voltage level of the position signal 121 fromthe sensor locating device 131 may vary from a lower voltage (e.g., 0V)to a higher voltage (e.g., 5V). The lower voltage corresponds to thesensor 132 being at one end of its travel path (i.e., at one end of thesample chamber 10), while the upper voltage corresponds to the sensorbeing at the opposing end of its travel path (i.e., at the other end ofthe sample chamber). A voltage halfway between the lower and highervoltages corresponds to the mid-point of the sample chamber. Thus, insuch embodiments, the voltage level from sensor 132 correlates tolocation/distance along the length of the sample chamber.

In accordance with a preferred embodiment, the magnet 76 is installed inor on the piston 20 such that the magnet's north pole is pointed in thex-direction. The strength of the magnetic field emanating from magnet 76varies with respect to location along the line of travel in thex-direction of the magnetic field sensor 132. The x- and y-components ofthe magnetic field from magnet 76 are depicted in FIGS. 6 a and 6 b,respectively. The x-component of the magnetic field depicted in FIG. 6 ashows that the x-component of the magnetic field has an absolute valuethat is a maximum at x=0, which corresponds to the location of themagnet 76 and thus the piston 20. That is, as the magnetic field sensor132 sweeps from one end of the sample chamber to the other, the detectedmagnetic field (x-component) is a maximum (in absolute value) when thesensor 132 is adjacent the magnet 76. The magnetic field sensor 132produces an electrical signal 123 that is provided to the control unit110 which thus is able to monitor the magnitude of the sensor's signal123 to detect the peak in the detected magnetic field. Once the magneticfield peak is detected, the control unit 110 reads the position signalfrom the sensor locating device 131 to determine the sensor's positioncorresponding to the peak of the magnetic field. From that position, thecontrol unit 110 is able to determine the distance Dl the piston 20 iswithin the sealed chamber from end 127.

FIG. 6 b depicts the y-component of the magnetic field from magnet 76.In some embodiments, the magnetic field sensor 132 has sensitivity inthe y-direction instead of, or in addition to, the x-direction In suchembodiments, the control unit 110 can determine when the magnetic fieldsensor 132 is adjacent the magnetic 76, and thus piston 20, bydetermining when the y-component of the magnetic field crosses through 0at point 139. As explained above, when the control unit 110 determinesthat the sensor 132 is adjacent the magnet 76, the control unit 110reads the position signal from the sensor locating device 131 todetermine the sensor's position corresponding to the peak magneticfield. From that position the control unit 110 is able to determine thedistance D1 the piston 20 is within the sealed chamber from end 127.

Depending on whether a single-axis or multi-axis magnetic field sensor132 is used, the control unit 110 determines when the sensor 132 isadjacent the magnet 76 using the x-component of the magnetic field, they-component of the magnetic field, or a combination of both. If both thex- and y-components are used, the magnetic field sensor 132 provides twosignals to the control unit 110—one signal corresponding to eachmagnetic field component. The control unit 110 may, for example, use onesignal as confirmation that the other signal is accurately indicatingmagnet 76 location. Alternatively, the control unit 110 may average thetimes at which the control unit 110 determines the magnet location fromboth signals and determine the piston location using the position signal121 from the sensor locating device 131 corresponding to the computedaverage time value.

In accordance with various embodiments, the sample chamber iscylindrical. The volume of a cylinder is computed as Dπr² where D is thelength of the cylinder and r is its cross-sectional radius. Referring toFIG. 5, the collected fluid sample is in the portion 77 of the samplechamber between the piston 20 and sample chamber end 127. The volume ofthat portion and thus the volume of the fluid sample is (D1)(πr₁ ²)where r₁ is the cross-section radius of sample chamber 10. The radius r₁is known ahead of time and distance D1 is determined by the control unit110 by reading the magnetic field sensor 132 and sensor locating device131.

FIGS. 7-10 illustrate various method embodiments. The order of theactions depicted in these methods may be as shown in the figures or maybe different from that shown. Further, not all of the actions arenecessarily performed sequentially. Instead, two or more actions may beperformed in parallel.

In FIG. 7, well drilling begins at 202. The drilling operation maycomprise any type of drilling such as vertical, deviated or horizontaldrilling, multi-lateral drilling, or conventional drilling orunder-balanced drilling. During the drilling phase, various types oftests may be performed using wireline, measurement while drilling (MWD),logging while drilling (LWD), etc. The testing described herein can bewith any such type of testing paradigm.

At 204, a sample chamber 10 is weighed at the surface and thus before afluid sample is collected. The chamber's weight is recorded into thetest unit 100 (e.g., the control unit 110). At 206, the sample chamber10 is placed into the test system 100 and the piston's position isdetermined and also recorded into the test system 100 (e.g., in storage114). This “initial” piston position thus is the position before a fluidsample is taken. At 208, the sample chamber 10 is lowered down the wellbore and a fluid sample is collected at 210.

If a downhole pump 60 is used (e.g., as with the sample chamberembodiments of FIGS. 3 and 4) and such a pump includes a filling sensoras explained above, at 212 the pump measures or estimates the samplevolume which may be transmitted to the surface via wireline or mudpulsecommunication techniques as noted above. The sample chamber 10 isbrought up to the surface at 214 and, at 216, the sample chamber 10 isagain weighed. The post-sample weight of the sample chamber 10 isrecorded into the test system. The difference in the before and afterweights of the sample chamber corresponds to the weight/mass of thefluid sample itself.

At 218, the sample chamber 10 is loaded into the test system and, at220, the hydraulic line 122 is connected to the sample chamber. At 222,the control unit 110 determines the mass of the fluid sample by, forexample, subtracting the initial (no sample) weight of the samplechamber from the weight of the chamber containing the fluid sample. Themass may be stored in storage 114. At 224, the control unit 110determines the in situ density of the fluid sample in the sealed samplechamber.

FIGS. 8 and 9 illustrate two embodiments for determining the in situdensity of the fluid sample. At 226, the control unit 110 determines thecompressibility of the fluid sample and FIG. 10 illustrates a techniquefor determining compressibility.

FIG. 8 illustrates an embodiment 224 for determining in situ density.This embodiment is particularly useful if a downhole pump was used tofill the sample chamber. At 242, the control unit 110 retrieves thesample volume as measured or estimated by the downhole pump 60 andtransmitted to surface as explained above. At 244, the control unitretrieves the sample mass as well. At 246, the control unit divides themass by the volume to compute in situ density.

FIG. 9 illustrates an alternative embodiment 224 for determining in situdensity, particularly if downhole pump 60 is not used or, if a pump isused without the ability to measure or estimate sample volume. At 252,the control unit 110 causes the magnetic field sensor 232 to beginsweeping across the outside of the sample chamber 10 to determine theposition of the piston 20. The position of the piston 20 informs thecontrol unit 110 as to the distance D1. Using distance D1, at 254 thecontrol unit 110 computes the in situ sample volume. In someembodiments, in situ volume is based on both the initial piston position(FIG. 7, 206) before the fluid sample is collected and the final pistonposition (252). Specifically, the distance D1 is computed to be thedifference between the initial and final piston positions. Computing thedifference in initial and final piston positions is useful if a gas wasincluded in the compartment in which the sample fluid is subsequentlycollected—the total volume of the compartment containing the samplefluid is the volume of both the sample fluid and the volume of theinitial gas and thus should be compensated for the volume of the gas forgreater volume accuracy.

At 256, the control unit 110 computes the sample volume as explainedabove. At 258, the control unit 110 retrieves the sample mass fromstorage 114 and, at 260, the in situ density is computed by, forexample, dividing the sample mass by the sample volume.

FIG. 10 provides a method embodiment 226 in which compressibility of thefluid sample is determined. In general, the piston 20 is moved withinthe sample chamber 10 to various positions thereby applying differentpressures on the fluid sample. The volume of the fluid sample isdetermined at each pressure setting. At 272, the control unit 110preferably causes the magnetic field sensor to repeatedly sweep back andforth across the outside of the sample chamber 10 along the x-direction.At 274, the control unit 110 asserts a signal to the hydraulic unit 120to cause the hydraulic unit thereby to incrementally increase thepressure in the hydraulic line 122. Referring briefly to FIG. 5, thehydraulic fluid in the hydraulic line 122 is in fluid communication withspace 79 behind the piston 20. Thus, an increase in pressure in thehydraulic line 122 is also asserted against the piston 20. Until thepressure of the hydraulic fluid in space 79 exceeds the pressure of thefluid sample in space 77, the piston 20 will not move toward end 127 ofthe chamber 10. Thus, at 276, the control unit 110 monitors the signals121 and 123 from the linear position device 130 to detect movement ofthe piston 20. The pressure at which the piston 20 begins to move isreferred to as the “opening pressure.” Once the opening pressure isreached, increasing the hydraulic pressure further causes the piston 20to move toward end 127 of the chamber 10. Further, for each suchhydraulic pressure in excess of the opening pressure, the piston willmove to and stop at a certain point within the chamber 10; that point isthe location at which the hydraulic pressure substantially equals thepressure of the fluid sample (which itself experiences an increases inpressure as the piston 20 increasingly compresses the fluid sample).

If piston movement is not detected at 276, then control loops back to274 at which the hydraulic pressure is again incremented (e.g., inincrements of 100 psi). Once piston movement is detected, however,control passes to 278 at which the control unit 110 records the pressureof the hydraulic line as measured by pressure sensor 124. Becausehydraulic pressure of line 122 is substantially equal to the fluidsample pressure, the pressure measured by pressure sensor 124 is alsothe pressure of the fluid sample.

At 280, the control unit also measures the position of filling piston 20thereby to determine distance D1 associated with piston 20. Preferably,distance D1 is computed as the difference between the newly measuredpiston distance and its initial distance before the fluid sample wascollected (FIG. 7, 206). At 282, using newly determined distance D1 andthe known radius of the cylindrical sample chamber 10 (or, in general,known cross-sectional area of chamber if the chamber has a shape otherthan cylindrical), the sample volume is computed by control unit 110 asthe cross-sectional area times Dl.

At 284, the control unit 110 computes the fluid sample compressibilityfor the current sample pressure. Compressibility is defined as thefractional change of volume due to changes in pressure for a constanttemperature and is estimated using the following equation:

${Comp} = {{- \frac{1}{V}}\left( \frac{\partial V}{\partial p} \right)_{T}}$

where V is the total volume of the sample chamber 10 and T is a constanttemperature. The control unit 110 calculates and stores and/or displaysa compressibility value for each pressure measured at 278.

At 286, the control unit 110 determines whether a stopping condition ismet. In some embodiments, the stopping condition may comprise athreshold pressure level (e.g., 10,000 psi). If the stopping conditionhas not been reached, then control passes to 288 in which the controlunit 110 causes the hydraulic unit 120 to incrementally change (e.g.,increase) the hydraulic pressure in hydraulic line 122 to further movepiston 20 to thereby further compress the fluid sample in space 77. Theincrement in pressure may be in increments of, for example, 500 psi. Theprocess loops back to 278 for another pressure measurement, and so on.If, at 284, it is determined that the stopping condition has been met,then the process stops at 290.

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.

1. A method, comprising: changing a hydraulic pressure applied to apiston within a sealed sample chamber, said sample chamber containing afluid sample; measuring said hydraulic pressure with a pressure sensorexternal to said sample chamber; measuring, with a magnetic sensor, adistance associated with said piston; and computing compressibility ofsaid fluid sample contained within said sealed sample chamber based onthe distance and the measured hydraulic pressure.
 2. The method of claim1 further comprising repeating each of changing the hydraulic pressure,measuring the hydraulic pressure, and measuring the distance associatedwith said piston at each such pressure.
 3. The method of claim 2 whereincomputing compressibility comprises computing compressibility based on aplurality of the measured distances associated with the piston atcorresponding pressures.
 4. The method of claim 1 wherein beforemeasuring the hydraulic pressure, increasing the hydraulic pressureuntil movement of the piston is detected.
 5. The method of claim 1further comprising lowering said sample chamber downhole before changingthe hydraulic pressure, measuring the hydraulic pressure, measuring thedistance, and computing compressibility.
 6. A method, comprising:weighing a sample chamber before receiving a downhole fluid sample;collecting a downhole fluid sample in said sample chamber; weighing saidsample chamber with said fluid sample; computing mass of said fluidsample based on the weight of the sample chamber before and aftercollecting said fluid sample; determining volume of said fluid sampleusing one of a magnetic field sensor that determines the location of apiston contained within the sample chamber and a downhole pump fillingsensor that determines the volume of the sample collected while the pumpfills the sample chamber; and computing in situ density of said fluidsample based on said mass and volume of said fluid sample.
 7. The methodof claim 6 wherein the piston comprises a magnet and wherein determiningthe volume of said fluid sample comprises sweeping the magnetic fieldsensor outside of and across a length of, said sample chamber to locatethe piston and measure a distance associated with the piston.
 8. Themethod of claim 7 wherein determining the volume comprises computing thevolume based on the distance.
 9. A system for testing a fluid samplecontained within a sealed sample chamber containing a fluid sample,comprising: a control unit; a hydraulic unit having a hydraulic line,said hydraulic unit electrically coupled to said control unit; apressure sensor coupled to said hydraulic line and to the control unit,said pressure sensor producing a pressure signal proportional to thepressure of said hydraulic line; and a linear position device coupled tosaid control unit, said linear position device comprising a movablemagnetic field sensor sensitive to a magnetic field emanating from amagnet contained in said sample chamber, said linear position deviceproducing a position signal indicative of a position of the movablemagnetic field sensor; wherein said control unit: causes said hydraulicunit to change pressure in said hydraulic line to thereby move thepiston within the sealed sample chamber; receives the pressure signalfrom said pressure sensor and the position signal from said linearposition device; and computes compressibility of the fluid sample basedon the pressure signal and the position signal.
 10. The system of claim9 wherein the movable magnetic field sensor produces an outputelectrical signal that is provided to the control unit and used by thecontrol unit to locate the position of magnet in the sample chamber. 11.The system of claim 9 wherein the control unit computes volume of thefluid sample based on the position signal from the linear positiondevice.
 12. The system of claim 9 wherein the control unit determines insitu density of the fluid sample in the sealed sample chamber based onthe position signal from the linear position device.
 13. The system ofclaim 9 wherein the control unit computes compressibility of the fluidsample also based on multiple positions of the magnet within the samplechamber at multiple pressures.